The present invention relates to hard metal bodies having increased wear resistance and to a method for producing them. The hard metal bodies include at least one of the binder metals iron, cobalt and nickel and at least one carbide of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten and contain nitrogen in their surfaces.
It has long been known that hard metal bodies can be formed from at least one binder or bonding metal of iron, cobalt and nickel and at least one hard metal refractory carbide of at least one of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. The hard metal body generally is formed by uniting a powdered form of the hard metal carbide by compression with the binding metal, followed by sintering. In some instances, the forming of the hard metal body includes a presintering step at low temperatures (e.g. about 800.degree. C.) to give it sufficient strength to be ground or cut to more complex shapes than can be formed by pressing. Final sintering is then carried out at a much higher temperature, specific for each composition. During the final sintering process, whether or not it follows a presintering step, the product generally receives its final shape and dimensions and the resulting sintered product is a molded, shaped, hard metal body which often is referred to as a cemented carbide. The hard metal bodies possess great hardness and find wide application in metal turning and cutting tools which are hard enough to permit high turning and cutting speeds in rock or metal.
Increasing demands have been placed on hard metal bodies and there has been a continuing search to provide hard metal bodies having still greater wear resistance. To this end, there has been produced hard metal bodies comprising a core of a shaped, hard metal body formed from a hard metal carbide and bonding metal as described above and a surface coating of a hard material on the core. In particular, it is known to provide hard metal bodies with a surface coating of hard material, such as with a coating of carbides, nitrides, carbonitrides, borides and/or oxides so as to significantly increase their hardness at their surfaces. The surface coating of hard material generally is formed by deposition on the core of the hard metal body during a separate process step. For example, deposition from the gaseous phase according to the chemical vapor deposition (CVD) process is a preferred method for forming a surface coating on a hard metal body. The application of one or more surface layers has been effected, for example, as described in German Offenlegungsschrift No. 24 33 737 corresponding to U.S. Pat. No. 3,999,954 and German Offenlegungsschrift No. 25 25 185 corresponding to U.S. Pat. No. 4,019,873, by means of a CVD process (chemical vapor deposition) or a PVD process (physical vapor deposition), in a separate process step. These coated hard metal bodies are produced by forming a coating on an already formed hard metal body and thus have the drawback that an additional process step for the coating is required in their manufacture. A further drawback of these coated hard metal bodies is their low thermal stress resistance. Due to the different coefficients of expansion of the basic body substrate (core of hard metal body) and the vapor-deposited surface material, intensive heating produces stresses which may in the end result in the surface coating coming loose from the basic body.
In view of these facts, the hard metal bodies produced according to the above-mentioned processes can withstand high toughness stresses only conditionally. The easy detachment of such vapor-deposited coatings generally leads to a limitation of the maximum possible thickness of the surface layer of 15.mu..
Austrian Patent No. 314,212 claims a process according to which the alloys are treated for the duration of the sintering process under a gas pressure of 2 to 500 bar, preferably 20 to 200 bar, in gases which can either be inert gases or which are required, inter alia, for the alloy formation. The hard metals produced according to this process may, however, have an unfavorable structure configuration. A further drawback of this process is the increased sintering temperature.
Austrian Patent No. 331,049 discloses nitration of the surface of hard metal bodies by diffusing nascent nitrogen into the surface of the molded body. This nascent nitrogen is produced by splitting ammonia at 550.degree. C. or by catalytically splitting molecular nitrogen at 1000.degree. C. Nitrogen enrichment of the hard metal surface is also possible according to Austrian Patent No. 331,049 by effecting treatment in a molten sodium cyanide and sodium cyanate salt bath or a potassium cyanide and potassium cyanate salt bath at 550.degree. to 600.degree. C. Such nitrogen enrichment under normal pressure must be effected in a second process step after formation of the hard metal body, which is a drawback.